Mar. 3, 1923 
Further Studies of Corticium vagum on Potato 
767 
with little difficulty by the irregular size of the plants, yet “misses” 
were infrequent and many of the control plants were equal in size to those 
from treated seed. Fewer growing points were destroyed than was noted 
in 1918, while few, if any, secondary stems were seriously attacked. The 
lesions on the whole were much less severe than those of the previous 
year, as is shown by the fact that no reduction occurred either in the 
stand or in the number of stems per hill in the diseased as compared 
with the healthy hills. The relative severity of the disease is further shown 
by the smaller decrease in the yield from the diseased hills, as indicated 
both in the weight and number of tubers per hill. In every respect 
the action of the fungus was definitely less severe during 1919 than 
during 1918. The comparative results are shown in Table III. 
As stated, the most important uncontrolled factors operating during 
the two seasons of 1918 and 1919 were soil temperature and soil moisture. 
The precipitation was much lighter during the first four weeks after 
planting in 1918 than during the same period of 1919, as is shown in 
Table IV. As shown in Table V, during June, 1918, 2.49 inches fell, as 
compared with 5.69 inches during June, 1919. For the second, third, 
and fourth weeks of growth in 1918 and 1919 the rainfall was 2.24 and 
4.23 inches, respectively. It should be noted, however, that during the 
month of May, 1918, 9.78 inches fell, while 4.08 inches of rain fell in May, 
1919. This early rainfall no doubt influences greatly the amount of 
moisture in the soil during this early part of the crop development. 
Table V .—Monthly mean of daily temperature and total monthly rainfall for Hancock t 
Wis. y during the potato-growing season of igi8 and IQIQ 
Month. 
Average daily mean air 
temperature. 
Total rainfall. 
1918 
1929 
1918 
1919 
May. 
16. 0 
19. I 
21. 25 
22. 7 
12.75 
13 -1 
22.0 
23. 2 
19. 0 
9.78 
2.49 
I. 92 
3. 12 
I. 31 
4. 08 
5-69 
2. 13 
2.3I 
June. 
August. 
September. 
By far the greatest damage to the crop occurred during the season of 
least rainfall. While little positive evidence is available as to the exact 
effect of moisture on the pathogenic action of Corticium vagum on the 
potato, it has been generally assumed to be more virulent in relatively 
damp soil. It is hardly probable that the decreased rainfall during 1918 
functioned as an important factor in the greater damage done by the 
fungus during that year. It is suggested in this connection that the low 
moisture content of the soil during 1918 might have acted as a retarding 
factor in the growth of the young shoot through the soils, thus contribut¬ 
ing to the severity of the disease. No indication of such retardation, 
however, was noted. In fact, the crop on the plots in general exhibited 
greater vitality during the early season of 1918 than during the same 
period of 1919. 
In view of the known facts the relation of soil temperature to the varia¬ 
tion in the severity of the disease during 1918 and 1919 appears more clear. 
